1. Field of the Invention
The present invention relates to a semiconductor device having a differential negative resistance in response to an electric field applied thereto.
2. Description of the Prior Art
A semiconductor device exhibiting a differential negative resistance (referred to as negative resistance) is used in a variety of circuits, for example, switching circuits, oscillation circuits, amplifier circuits, detector circuits, frequency mixer circuits or the like. It has been desired that such a semiconductor device can flexibly be designed according to any specification of a circuit to which the semiconductor device is applied.
Various types of negative resistance devices have been produced. Some notable devices of this type recently proposed are Gunn effect devices, tunnel diodes, perfectly periodic superlattice structure devices, or the like.
The Gunn effect device is made of direct-gap compound semiconductor such as GaAs or InP. In this device, the Gunn effect is determined solely by the energy band structure of the material, so that it is difficult to modify any characteristic of the Gunn effect artificially. Therefore, the Gunn effect limits the designable range for an electric field providing negative resistance, a current density, a size of the Gunn effect device, an electron density, and so on. For this reason, it is very difficult to flexibly design the Gunn effect device in accordance with any specifications of an electric circuit to which the Gunn effect device is applied.
A tunnel diode utilizes the tunnel phenomonon of electrons through the p-n junction to which impurities of high concentration are doped. Therefore, the negative resistance characteristic is determined solely by the energy band structure of the semiconductor to be used. This feature extremely restricts a design freedom of the tunnel diode and thus it is difficult to design the tunnel diode in accordance with any specifications of an electric circuit to which the tunnel diode is applied.
A semiconductor device with a perfectly periodic superlattice structure is disclosed in U.S. Pat. Nos. 3,626,328 and 3,626,257 by L. Esaki et al.
Such a superlattice structure will be given with reference to FIGS. 1 and 2. FIG. 1 schematically illustrates a potential 1 formed dependent on the band edge energy for electrons produced by the perfectly periodic superlattice structure (referred to merely as superlattice hereinafter), the allowed bands 2 in the superlattice based on the potential 1 (referred to as a miniband), and forbidden gaps 3 in the superlattice (referred to as minigaps). In FIG. 1, the abscissa represents a distance x and the ordinate represents an energy E for electrons.
Further, the minibands 2 and the minigaps 3 formed in connection with the potential 1 shown in FIG. 1 are schematically illustrated. In the figure, the period of the potential 1 is designated by d, and the height or amplitude of the potential 1 by V.sub.0. Further, the minigaps 3 and the minibands 2, as illustrated, are only the first to third ones from the bottom, for the simplicity of illustration. When an electric field is applied to the superlattice, the potential 1, the minigaps 3, and the minibands 2 in FIG. 1 are varied as shown in FIG. 2.
In FIG. 2, the potential 1, the minibands 2, and the minigaps 3 are tilted down in the X direction due to the application of the electric field. The electron e at the bottom edge of the first miniband 2, or at point P1, moves back and forth between points P1 and P2 of the first miniband 2. In order that this oscillating motion causes the negative resistance in the superlattice, all of the electrons in the superlattice must repeat the oscillation motions simultaneously, in the same direction, and in a synchronizing manner. For this purpose, it is necessary to minimize the scattering by the lattice vibrations and the doped impurities. Further, the potential 1 must be designed so as to reduce the number of electrons which tunnels through, for instance, the second minigap 3 to reach point P3 in FIG. 2. In order to obtain the negative resistance in the superlattice in this manner, some measure must be taken to minimize such scattering and tunnelling of electrons. This makes the device design remarkably difficult.
In the case of injecting electrons into the superlattice, it can not always be assured that such electrons efficiently enter the superlattice to completely penetrate through the superlattice, even if it is possible that electrons with certain energy can be present in the superlattice. Actually, according to the simulation conducted by the inventors, the perfect reflection of electrons was easily realized, whereas the penetration of electrons therethrough is low in efficiency.